* feat(Transformations): added Transformations abstraction & handling in walk_forward_train() & inference()
* fix(WalkForward): use Dataframes to call Transformation.fit_transform()
* feat(WalkForward): restored option for models to recieve unscaled data
* fix(Transformations): output DataFrame as expected
* fix(Tests): missing new property
* feat(Models): added lightGBM, moved other models to separate files
* feat(Models): added non-working statsmodel wrapper
* fix(Models): added work-in-progress comment to StatsModels